The first portion of the background is most closely related to a sonic and dual stage gas inlet valve. In the case of the ECI conversions systems for 2 stroke locomotive engines, a system called low pressure direct injection (LPDI) is used where the natural gas is injected directly into the cylinder during the compression stroke. What this leads to is a mixing challenge where the air and fuel have limited time to mix as the piston rises up to top dead center right before ignition.
This mixing challenge is why SwRI on their single cylinder development EMD 710 engine decided to do premixing of the air and fuel even though it would not be practical on an ‘in service’ engine as too much unburned fuel would blow through the cylinder into the exhaust while scavenging.
The in cylinder mixing issue can make prechamber operation difficult if a rich pocket of air and gas gets pushed into the prechamber which already has excess fuel in it. In this instance, the prechamber will misfire and there will be no combustion for that stroke. For this reason. ECI installed ‘jet caps’ on the first iteration spark ignited prechamber (SIP) system on the Napa Valley Wine train. The jet cap is an additional cap fixed over the end of the main Gas inlet valve (GIV). The GIV had a poppet valve at the end that controlled the flow of fuel gas into the combustion chamber. With the ‘jet cap’ in place, after the gas flowed thru the GIV body and past the poppet valve, it then had to flow through a small orifice at the end of the ‘jet cap’. This addressed several issues, all the gas was converged into one flow stream that now had higher velocity and was pointed away from the prechamber.
Another difference between the ECI kit and the system tested at SwRI is that the ECI system has to operate at very high Lambdas. Lambda is the ratio of the actual air/fuel ratio divided by the stoichiometric air/fuel ratio. Typical 4 stroke diesel engines operate at Lambdas around 1.9 at low load to 1.4 at full power. The SwRI single cylinder development engine didn't have to operate below 50% power. At low loads, an EMD 2 stroke locomotive operates at Lambda's above 3 and at idle the Lambda can exceed 4. At these very high lambdas it would require a larger prechamber that will produce fewer NOx emissions and have a lower thermal efficiency.
A solution to the very high Lambda value is to restrict inlet flow with a throttling system at low loads. This will allow operating the engine all the way from idle to full load with smaller volume prechambers that put out less NOx emissions and operate at higher thermal efficiency.
In a uniflow 2 stroke engine, scavenging is a process of blowing inlet air over the top of the piston at bottom dead center. This entering intake air pushes the spent combustion gasses out through the open exhaust ports at the top of the cylinder. The amount of in cylinder air motion and mixing as the piston rises in the compression stroke is proportional to how much velocity the inlet air carried in with it due to excess intake air box pressure. When the inlet is throttled to help reduce the low load air fuel Lambda, a large portion of this mixing energy is lost.
It is possible to reduce the inlet air box pressure to a low enough value that not enough inlet air enters to thoroughly scavenge the cylinder and some amount of exhaust gas will remain in the cylinder when the exhaust valves close. This effect can be desirable or have negative effects. This left over combustion gas is much hotter and less dense than the incoming air, so the resulting in cylinder air mass will now be lower and the average in cylinder temperature will be hotter at the beginning of the compression stroke. This has the double effect of both lowering the Lambda for easier combustion with less ignition energy using a smaller prechamber, and also faster and more efficient combustion because the compressed air fuel mixture is already much hotter at ignition.
This is referred to as internal exhaust gas recirculation (EGR) where exhaust gas is purposely left behind to achieve these effects. In a uniflow 2 stroke, the downside of this is much less air velocity at intake port closing. This lowered in cylinder velocity and mixing energy reduces the amount of air and fuel mixing when the natural gas is injected at low loads.
A supersonic injector for gaseous fuel engines as described in U.S. Pat. No. 6,708,905 would be a solution that offers improved mixing and a bonus of lower temperature gas when injected. This particular device has two drawbacks. First it has many machined parts with complicated features that will be costly. Second, the design has a built in cavity where residual natural gas will be compressed into and remain unburned during the combustion event. Most of the compressed gas in this cavity will become methane exhaust emissions. This release of unburned methane is both a pollution emissions problem and an energy efficiency problem.
What is desired is an economical and practical way to achieve the benefits of a high velocity and focused sonic injection nozzle without the added cavity for residual unburned methane, better mixing in the combustion chamber of a natural gas engine with direct gas injection which would allow operating a uniflow 2 cycle engine to be throttled past the point that internal EGR effects are improving combustion.
The second portion of this background is most closely related to continuous water injection for ECI converted engines. Water injection has been used in engines to reduce engine knock at higher power levels as far back as World War 2. It was commonly used to allow aircraft engines to generate extra power during takeoff and other possible events that needed the most power possible.
It has also seen some use in racing applications, typically in sprint type racing where the time duration of full power and water injection use is limited, thus avoiding a bulky and heavy water storage system.
There are several issues that make water injection not worth the effort of implementation in most mobile applications; one is the volume and weight of the consumable water and second is the need to refill the container that would store it. Once these issues are overcome, then there is the environmental issue of keeping the stored onboard water from freezing when the vehicle is not in operation.
Another issue is the challenge of corrosion to the hardware that would be used to inject it, especially if the injector is designed to open and close rapidly for each cylinders combustion cycle.
Finally is the corrosion issue as related to any other parts. If after shut down an injector would leak water into the engine cylinder during engine storage, that cylinder will have internal corrosion and suffer significant maintenance issues.
Several Papers have indicated that direct injection of water into the engine cylinder has several advantages in addition to reducing engine knocking. SAE paper 2009-01-1925 Effect of In Cylinder Water Injection Strategies on Performance and Emissions of a Hydrogen Fueled Direct Injection Engine is one good example. In this paper it is indicated that water injection both lowered NOx emissions and increased the indicated thermal efficiency when the water injection happened during the compression stroke. This effect was much less when the water was injected during the intake stroke on the four stroke engine tested.
When converting a diesel engine over to operate on natural gas, the compression ratio is typically reduced. If it wasn't reduced the engine may be limited to only generation of 60% of its original diesel operation rated power. The addition of water injection could allow the retention of higher compression ratios.
The third portion of this background is most closely related prechamber cylinder deactivation on spark ignited prechamber EMD engines. Both the roots blown and turbocharged EMD engines would be good candidates for cylinder deactivation. Currently ECI used skip firing in their Spark Ignited Prechamber systems to improve combustion at very low loads where the engine operates very lean. In skip fire, the engine controller will skip actuating the main injector for a certain cylinder. This will cause the other cylinders to have to operate at a higher power to make up the lost power from the deactivated cylinders. When operating at higher power the other cylinders will need more fuel to generate it and this increase in fuel to those cylinders is what decreases how lean those cylinders are before ignition which generates higher heat release rates making the combustion events more consistent, and efficient. The control system has a strategy to alternate the deactivated cylinders to prevent any one cylinder from becoming significantly cooler than the others and also to prevent lube oil build up in that cylinder.
To keep the system simple, only the main gas injector is turned off for the cylinders that are skipped. All of the engine prechambers are still fed natural gas and the spark plugs are still fired. In the case where the prechamber supply pressure can be held constant over the entire engine operating range, the prechamber fuel supply system consists of only a single mechanical pressure regulator with a fixed setting.
Because the prechamber is still fed fuel, but the main chamber is not, there is no guarantee that the mixture in the prechamber is being burned when a cylinder is deactivated, even when the spark plug is still being fired. A portion of the fuel burned in the prechamber during normal combustion was not injected by the prechamber fuel system, but was brought in from the main chamber. When the cylinder is deactivated the air pushed into the prechamber by the piston will not have any fuel so the overall mixture in the prechamber may be too lean for the spark to burn. This is greatly dependent on engine speed and load while being skip fired. Because skip fire happens at low load it's likely that the extended time the system gets to fill the prechamber offsets this deactivated cylinder issue, but at the same time the operating cylinders are running richer and having the deactivated cylinders prechambers rich enough to fire may make the activated prechambers too rich causing misfires or combustion instability.
With these issues in mind, prechambers that are fed fuel in deactivated cylinders are likely to generate more NOx or HC or both. The generation of Non-Methane HC emissions is especially problematic as after the spark plug initiates combustion in the prechamber some unburned natural gas is pushed out of the prechamber into the main chamber before it is burned inside the prechamber.